Ring laser sensors are well known in the art and some examples are specifically described in U.S. Pat. Nos. 3,627,425, 3,373,650, and 3,390,606 which are assigned to the same assignee as the present application. Sensors such as these employ counterpropagating laser beams traveling about a closed-loop path. The frequency difference between the counterpropagating laser beams is directly related to the rotation of the sensor. The sensor commonly employs an optical system for projecting a portion of each of the counterpropagating laser beams at a slight angle with each other so as to create an interference fringe pattern which is projected on a light responsive device such as a photodetector for monitoring the intensity variation of the interference fringe pattern.
Detectors of the prior art employ at least one photodetector at a fixed spatial position relative to the interference fringe pattern for providing an output signal indicative of the intensity of that portion of the interference fringe pattern projected thereon. The output of the photodetector is subsequently amplified and passed through a circuit means, for removing any DC component, and presented to one input of a comparator. The other end of the comparator, is normally connected to a reference potential near ground. The output of the comparator provides a square wave with positive going and negative going edges coincident with positive going and negative going zero crossings of the comparator signal input. The comparator output is connected to a positive edge detector and a negative edge detector which converts the comparator output into a series of corresponding pulses. In such a system as just described, the interval between two consecutive pulses represent an interference fringe change of one-half of the fringe spacing of the projected interference fringe pattern.
In the art of ring laser gyros, one-half of a fringe spacing change corresponds to a .pi. radian phase change between the counterpropagating laser beams. The total value of fringe change from some reference point in time corresponds to a specific angle of rotation of the sensor dependent upon the sensor's scale factor which is a function of the sensor's closed-loop path. Thus, counting of the number of pulses or fringe changes provides a system for obtaining angular rotation of the sensor. A detector system as just described is shown, at least in part, in U.S. Pat. No. 3,627,425.
For precision navigational systems, improved resolution greater than one-half a fringe spacing is desired. This may be obtained by providing a plurality of detectors responsive to the same interference pattern at different points in between a complete fringe spacing. For example, if two detectors are provided which are separated by one-quarter of a fringe spacing, appropriate circuitry can obtain a series of pulses representative of a change of one-quarter of a fringe. Nevertheless, in precision navigational systems, it is desirable to obtain an angular rotation measurement upon a request of a measurement command signal. Of course, this signal does not occur coincident with a detector output pulse, therefore, interpolation is required for enhanced accuracy.
It is the object of the present invention to provide an apparatus for measuring fringe changes of a varying interference fringe pattern upon a measurement command signal with substantially enhanced resolution and in which does not require a large number of detectors.